Optical resonators such as optical microrings are very sensitive optical sensor types that use the evanescent electromagnetic field travelling there through for scanning a surface. It is very difficult to produce multiple microrings with identical resonance properties, for example, the controlled spectral definition of a resonance peak with an accuracy in the picometer or sub-picometer region. One of the reasons why the production of identical microrings is so difficult is that the nanostructuring for producing said microrings is highly-dependent on the temperature dependency of the optical materials.
In order to scan resonance properties of different materials, passive reference resonators have been used. Examples for such resonators can be found in the document WO 2011/000494, where the reference resonators are actively controlled through a very sensitive temperature adjustment arrangement so that no or very small resonance shifts occur in the reference resonators. Said control units are very complex and not cost-efficient.
Document U.S. Pat. No. 7,903,906 B2 discloses an optical sensor arrangement for measuring an observable including at least one light source for generating a first light component of a first frequency comprising a first mode and a second light component of a second frequency comprising a second mode orthogonal to said first mode, an optical resonator having differing optical lengths for the first and second modes within a frequency interval including the first and second frequencies, at least one of the optical lengths being variable depending on the observable and a dependence of the respective optical length on the observable being different for said first and second modes. The at least one light source is optically coupled to the optical resonator for feeding the first and second light components into the optical resonators. Furthermore, a detector unit is present which is coupled to the optical resonator for coupling out the light components. Differences for each light component are measured in the presence of a substance and a resulting overall difference is calculated. Document US 2006/227331 discloses an optical sensor arrangement for measuring an observable comprising at least one light source for generating a first light component of a first frequency comprising a first mode and a second light component of a second frequency comprising a second mode orthogonal to said first mode. An optical ring resonator is present which has differing optical lengths for the first and second modes within in frequency interval. Furthermore, the light source is coupled to the optical resonator, and the optical resonator is coupled to a detector unit.
Similar devices are shown in documents US 2004/146431 and Peter Lützow et al., “Integrated optical sensor platform for multiparameter bio-chemical analysis”, Optics Express, vol. 19, no. 14, 4 Jul. 2011.